Mirror object/group on X or Y axis

Hi :)

Hopefully somebody can help me out here.

I'm looking for a script that easily mirror and object/group on the X or Y axis. I know that there are scripts that already do this, but they are part of another pop-up. I just like to attach it as a simple button i can press.

I've tried some stuff in the script below but my scripting skills are not well.

Thank you :)